High-Q chiral metasurfaces for strong valley-specific emission and entangled photon-pair generation at room temperature

Spin is one of the most fundamental properties of elementary particles. Photons, possessing spin angular momentum (circular polarization), typically exhibit long spin coherence times but inherently weak coupling to their environment. Efficiently harnessing circularly polarized light for quantum information systems thus demands exceptional control and manipulation of chiral light-matter interactions (LMIs). Here, I describe high-Q-quality chiral metasurfaces to enhance linear and nonlinear chiral LMIs for realizing solid-state, optically addressable spin qubits and chiral quantum light emission. These metasurfaces achieve high-Q chiroptical resonances by breaking both in-plane inversion and mirror symmetries, resulting in nonlocal chiral quasi-bound states in the continuum (q-BICs). First, I will show how a scalable, large-area heterostructured Si-MoSe2 platform composed of such a high-Q chiral metasurface enables simultaneous control of valley-specific emission spectra and far-field radiation in MoSe2 monolayers. Our crystalline Si metasurfaces matching the A-exciton resonance of MoSe2 monolayers (~770 nm) yield Q-factors of several hundred, enabling robust valley-specific emission against excitation polarization states with a record-high degree of circular polarization of 0.5 at room temperature. Angle-resolved measurements confirm localized, valley-polarized emission near the Γ-point, indicating efficient directional control via chiral q-BIC modes. Second, I will discuss a high-Q AlGaAs metasurface for spontaneous parametric down-conversion (SPDC) to generate non-degenerate entangled photon pairs encoded with spin angular momenta (SAM). Numerical simulations show high Q-factors (> 7,200) and significant field enhancement (E/E0: 600) at both signal (1412 nm) and idler (1231 nm) emission wavelengths, giving rise to photon-pair generation rates reaching 10⁶ Hz—four orders of magnitude higher than conventional nanoantenna structures. Overall, our results demonstrate that high-Q chiral metasurfaces can strongly enhance chiral LMIs toward highly efficient room-temperature single-photon and SAM-encoded entangled photon-pair generation.